Artificial transmission line



Feb. 26, 1957 G- D. PERKINS ARTIFICIAL TRANSMISSION LINE Filed Feb. 14, 1946 ATTORNEY ARTIFICIAL TRANSMISSION LINE George D. Perkins, Salem, Mass., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application February 14, 1946, Serial No. 647,590

6 Claims. (Cl. 333-20) This invention relates to artificial transmission lines and more particularly to a delay line with discontinuous characteristics for imparting to a selected reflection of an input pulse an amplitude and form substantially equal to that of the input pulse while suppressing other reflections.

Various types of coding and counting circuits depend upon the addition of a series of timed recurrent pulses to achieve an amplitude sufiicient to trigger associated circuits. One means of providing for the addition of consecutive pulses is an artificial delay line, short circuited at its far end, which returns a first input pulse to its input end at the proper time and of the proper polarity to reinforce the next desired pulse. However, the delay line will present other reflections which may be undesirable.

The present invention provides a delay line with discontinuous characteristics which causes a predetermined reflection to reinforce a subsequent pulse and which suppresses other undesired reflections.

An object of this invention is to provide 'a delay line for delaying an input pulse and reflecting the pulse to its input end with proper polarity and at a proper time to reinforce a subsequent input pulse and for suppressing other undesired reflections.

Another object is to provide a coding circuit for adding two pulses having a predetermined time spacing for achieving an amplitude suflicient to trigger associated circuits.

These and other objects and features of this invention will become apparent upon consideration of the following detailed description when taken together with the accompanying drawings, where- Fig. 1 is a schematic diagram of an artificial transmission line embodying this invention; and

Fig. 2 illustrates the actual waveform observed at the input end of the line of Fig. 1 for a single input pulse.

In Fig. 1, the artificial transmission line comprises four L sections of line, of inductance L1 and capacitance C1 per section, connected in series between input terminal 10 and terminal 11, and a fifth L section of inductance L2 and capacitance C2 connected between terminal 11 and terminal 12 at the end of the line. Terminal 12 is shortcircuited to ground. The five sections need not be L sections; they could be 1r or T sections as well.

Let the characteristic impedance of the line between terminals 10 and 11 be Col and between terminals 11 and 12 be 202- Then provided, of course, that the components of the input pulses have frequencies low compared to the cut-otf frequency of the delay line.

The artificial transmission line of Fig. 1 then comprises essentially a first delay line of characteristic impedance Z01 in series with a second delay line of characteristic impedance Z02 which is short-circuited at its far end.

If Z02 is difieren-t from Z01, an input pulse transmitted down the line from input terminal 10 will be partially reflected at the point of discontinuity at terminal 11 and the remainder of the pulse will continue on to terminal 12 where it is reflected in turn. The coefficient of reflection for a pulse reaching terminal 11 is If Z02 is less than Z01 the reflection coefii-cient is negative, signifying that the reflected portion of the pulse reaching terminal 11 is reversed in polarity from the incident pulse. At the short-circuited end of the line at terminal 12, the reflection coeflicient is 1 so that whatever pulse reaches the end of the line is entirely reflected with reversed polarity. The input terminating impedance at terminal 10 is very high compared to the characteristic impedance of the line between terminals 10 and 11 between input pulses so that substantially total reflection takes place at the input end of the line.

The time of delay of a pulse in traversing a section of line is equal 'to the square root of the product of the inductance and capacitance for the section. Thus for a round trip of the four line sections between terminals 10 and 11, the time of delay is SVTTL Between terminals 11 and 12 there is "an additional round trip delay of 2 /L2C2.

With negative reflection coetficients at both terminals 11 and 12, it will be apparent that the effect on an input pulse will be first to reverse its polarity on consecutive reflections to the input end at terminal 11; second, to broaden its base on each consecutive reflection by "an amount equal to the round trip delay time between terminals 11 and 12; and third, to reduce the peak amplitude for consecutive reflections. The peak amplitude of a reflected pulse occurs when the portion reflected from terminal 12 overlaps the portion reflected from terminal 11. Therefore, the shape of reflections besides being a function of the reflection coeflicients at terminals 11 and 12 is also a function of the width 6 of the input pulse and the total delay time of the delay line. Also, as a practical matter, there is some resistance associated with the units making up the delay line which has the effect of attenuating the amplitude of reflection-s, and the at tenuation effects increase for consecutive reflections.

In an illustrative application of this invention, a first input pulse is applied to input terminal 10 of the delay line and the second reflection from terminals 11 and 12 retums to the input end with the same polarity and substantially the same amplitude as the first input pulse. The second reflection of the first input pulse is timed to coincide with a second input pulse so that the resultant amplitude is suflicient to trigger associated circuits for which the amplitude of one pulse alone i insufficient. The first and third reflections being of opposite polarity to the input pulses would cancel any input pulses with which they happened to coincide. The fourth and subsequent reflections have insuflicient amplitudes to reinforce any subsequent input pulses and to bring them to a triggering level.

In terms of the width 5 of input pulses applied to input terminal 19, the circuit of Fig. 1 can have the following approximate relationships in order to obtain the above results, 6=8 /L1C1, L2= /sL1, C2=10C1, and the spacing between input pulses equal to twice the round trip delay time between input terminal 10 and the short circuited end of-apractical line constructedasiabove described. The

positive *pulse 16 is'thefirst input pulse and is less than ideally shaped. The negative pulse 1'7is'the first reflection. The positive pulsef18 is the second reflection whose shape and amplitude"aresubstantially the same asthat of input pulsei16. The delay time between peaks'of input pulse 16 and second'reflection' 13 isequalto the predetermined time or recurrence'interval-between input pulse 16 and a succeeding input pulse. Thus for a series of input pulses like 16 with the proper predetermined separation time, the circuit of Fig. l accomplishes reinforcement of two successive input pulses to obtain an approximately double peak amplitude which is sufficient to trigger associated circuits. The third-reflection i9 is again negative and the, fourth refiectiontZi), although positive, is of greatly diminished amplitude, insufiicient as are all subsequent reflections, for reinforcing input triggers with which they may coincide to a level high enough for triggering.

Although there is shown and described only a certain specific embodiment of this invention in a discontinuous delay line, the many modifications possible thereof will be readily apparent to those skilled in the art. Therefore this invention is not to be limited except insofar as is necessit-ated by the prior art and the :spirit of the appended claims.

What is claimed is:

1. An artificial transmission line adapted to receive input pulses comprising, a first delay line having a delay time substantially equal to the Width of said input pulses, a second delay line having a delay time substantially onethird said delay time of said first delay line, and means coupling said first and second delay lines to produce a reflection coeflicient of substantially 0.775 therebetween, said second line being short circuited at its end remote from said first line to produce thereat a reflection coeificient of 1, whereby the second reflection of each of said input pulses is of the same polarity and substantially the same amplitude as each of said input pulses, and all subsequent reflections of the same polarity are substantially suppressed.

2. An artificial transmission line adapted to receive a series'of input pulses of predetermined width and recur rence interval comprising, four L sections "of artificial transmissionlineconnected in series, each'ofsaid four sections having a delay time-substantiallyone-fourth said width of said input pulses, and a fifth L section coupled in series with said four L sections, said fifth section having a delay time substantially one-third said-width of said input pulses, said fifth section'being short-circuited at its end remote from said four sections to produce thereat a reflection coefficient of 1, said four sections and said fifth section having diflerent characteristic impedances such'as to produce a. reflection coeflicient of 0.775 between them, said total delay time of all of said L sections being equal to one-fourth said predeterminedrecurrence interval of said input pulses, whereby the second'reflection of each of said input pulses reinforcesthe next succeeding input pulse'which is of the same polarity, each said second refiection being substantially the same amplitude as each of said inputpulses, and all subsequent reflections of the same polarity being substantiallyssuppressed.

3. A delay line circuit in which successive reflections of like polarity have unequal amplitudes comprising, a first delay line having first and second ends, said first delay line being adapted to. receive input pulses at said first end, theinput terminating impedance :at said first end in the interval between pulses being substantially an open circuit, a second delay line having first and second ends,

said second delay line being coupled at its first end to said second end of saidfirst delay line asa terminatingimpedance of said first delay line, saidsecondldelay line having a characteristic impedance difierent from that of said first delay line, said second delayline being terminated at its second end in a short circuit.

4. A delay line circuit in which successive reflections of like polarity have unequal amplitudes. comprising, a first delay line having first and second ends, said .firstdelay line being adapted to. receive'input pulses at saidfirst end, the input terminating impedance at saidfirstend. in the interval between pulses being substantially an open circuit, a second delay line havingfirst and second ends, said second delay line being coupled at its first end to said second endof said firstdelay. line as a terminating impedance of said'first 'delay line, said second delay line having a characteristic impedance and delay time different from the characteristic impedance and delay time or said first delay line, said second, delay line being terminated at its second end in a short circuit.

'5. 'A delay linecircuit'in which successive reflections of like polarityf have unequal amplitudes comprising a first delay line having first and second ends, said firstdelay line being adapted to receive input pulses of predetermined width at said first end, the input terminating impedance at said'first. end in the interval between pulses being substantially an open circuit, thedelay timeof said first delay line being related to the time width of said input pulses by a simple fraction, the value ofunitybeing considered as a fraction, a second'delay line having firstand second ends, said second delay line being coupled at its .first end to said second end of said firstdelay. line as .a terminating impedance ofsaid first delay line,.said secondtdelay line havinga delay time shorter than the delay time of said first delay line, the delay time of .said second delay'line being related to'the time width of said input pulsesby. a simple fraction, said second delay. line having a characteristic impedance differing by a substantial amount. from the characteristic impedance, of. saidifirst .delay line, said second delay linebeing terminated at its second end in a short circuit.

6. A delay line, circuit as in claim Sir/herein said second delay line has a characteristioimpedance which is lower than the characteristic impedanceof said first delay line.

References Cited in the'file of this patent UNITED STATES PATENTS 2,255,839 -Wi1son -Sept. 16,1941 2,266,154 lBlumlein Dec. 16,1941 2,420,302 .Darlington May 13, 1947 2,420,309 .Goodall -May 13,1947 

